HVDC (High Voltage Direct Current) underwater cables and methods of laying such cables have been used since 1950's. When direct current power cables are in question, there are two cables involved. The cables may be equal or there may be one primary conductor to transmit direct current and another conductor to provide a return path. Underwater power cables and fiber optical cables are used today in an increasing amount due to the increased need of power transfer from for example offshore energy sources, including offshore renewable energy plants, such as wind power plants. Also the length for power transfer cables is increasing since there is a need to interconnect different regional electrical transmission networks to allow global trading of energy. Areas, where energy is on the other hand needed and on the other hand produced, may also be remote from each other which further increases a need for safe power transfer.
The underwater power cables may be for example HVDC or HVAC (High Voltage Alternating Current)-cables. Further common underwater cables are nowadays the above-mentioned fiber optical cables. It is known in the art to lay underwater cables and e.g. GB 2244321 A shows a method of laying such cables. It is also known in the art to lay for example two HVDC single core cables together with at least one fiber optical cable or three HVAC-cables together with at least one fiber optical cable.
As further also shown by GB 2244321 A, laying of cables can be performed from a vessel in which the cables are unwound from storage reels mounted on the vessel. The cables are laid or installed simultaneously. To be able to perform simultaneous laying or installation, the cables are secured firmly together side by side to form a cable bundle before being guided overboard into the water and then on the sea bed. The cables are often secured by means of strong adhesive tape, wires, belts or ropes, hereinafter called for securing means, which keep the cable bundle together even during burial and after burial into the sea bed.
A problem arises when one of the cables in the bundle needs to be repaired. In this case, the bundle is buried in the sea bed and the entire bundle needs to be cut in order to be able to bring the faulty cable to the surface for repair. As a consequence, the repair effort is much larger than necessary for the faulty cable. For example, in case the bundle comprises two HVDC cables and one fiber optical cable, and only the fiber optical cable is faulty, the problem is the following. To repair the fiber optical cable, the entire bundle must be cut at the sea bed, taken to the surface and spare cables need to be inserted in between the ends of the cut cables. Thus, for the repair of the fiber-optical cable, not only fiber optical cable joints need to be installed but also four power cable joints need to be installed. This is extremely costly and time consuming.
Thus, there is a need to facilitate handling of underwater cables. Especially there is a need to facilitate repairing of individual cables.